The present invention relates generally to imprinting electrical patterns on substrates, and more particularly to an improved technique for screen printing fine lines of conductive paste on substrates, wherein the lines are better defined and have a more uniform height than screen printing by prior known techniques.
In the electronic packaging technology microminiaturized integrated circuit devices with thousands of circuits and often hundreds of I/O terminals are mounted on and electrically connected to circuitry of a larger support substrate. The support substrate frequently has mounted thereon a plurality of such integrated circuit devices which must be interconnected with each other an operative relationship. Also provided on the support substrate are large I/O terminals that are joined to the circuitry and which provide sufficiently large terminals to be joined to cooperating terminals on associated boards, cable connections or other connections that makes available the function of the devices to an overall machine, such as a computer.
As more circuitry was placed in the integrated circuit devices, the electrical circuitry in the support substrate became more complex and more miniaturized. These demands were met by using a multilayer ceramic substrate, as described in U.S. Pat. No. 4,245,273.
The MLC substrate is fabricated by preparing a slurry containing particulate ceramic material, an organic resin binder, and a solvent for the binder. The slurry is doctor bladed into sheets, the sheets dried, via holes punched into the sheets, circuitry patterns imprinted on the sheet, and the via holes filled with a conductive paste. The resultant sheets are assembled and laminated, and the resultant substrate sintered to burn off the binder and fuse the ceramic particles into a unitary element which contains an internal metallurgy system.
The via holes in the green ceramic and the circuitry pattern are conventionally deposited on the sheets by screen printing, wherein a mixture of a particulate metal, a vehicle, and a solvent, is deposited in openings in a mask placed over the sheet. Various apparatus for screen printing electrical patterns on substrates are described and claimed in U.S. Pat. Nos. 3,384,931, 3,416,440 and 4,068,994. As the circuitry patterns in the masks became more complex, metal tabs with a thickness less than the thickness of the mask were provided periodically in the openings defining the line patterns to perserve the integrity of the mask. Such tab elements in masks are described in IBM TDB October 1967 P. 1790.
As the deposited circuitry lines become smaller and more closely spaced, the tolerance of height and width of the screened lines become more critical. An open in the line or a short between closely spaced lines may render the substrate useless. Further, such defects may not be detected until it is tested after it has been sintered. At this point in time, a great deal of money has been invested in the substrate. The scrapping of the substrate at this stage is expensive.